Maleic anhydride is of significant commercial interest throughout the world and is extensively used in the manufacture of alkyd resins. It is also a versatile intermediate of chemical synthesis. Consequently, large quantities of maleic anhydride are produced each year to satisfy these needs. The production of maleic anhydride by the catalytic oxidation of benzene and butene is well-known, and until recently, the principal method employed for the manufacture of maleic anhydride was by the air oxidation of benzene in the presence of certain heavy metal oxide catalysts. However, because of the inherent toxicity of benzene fumes, the trend has been to eliminate the utilization of benzene as a feedstock and newer facilities tend to utilize butane oxidation processes.
In general, catalysts proposed for the oxidation of butane to maleic anhydride have been based upon vanadium and phosphorus. In U.S. Pat. No. 3,293,268 it is disclosed that the oxidation of butane to maleic anhydride can be performed in the presence of a phosphorus-vanadium-oxygen-containing complex catalyst. Though this catalyst is capable of oxidizing butane, it does not give sufficiently high yields. Yields of maleic anhydride of only 30 to 50 weight percent are reported. Various activators, stabilizers and promoters have been disclosed in the prior art to improve the yields of maleic anhydride. References include U.S. Pat. Nos. 3,867,411; 3,832,359; 3,888,886; 4,002,650; 4,147,661; 4,149,992; 4,151,116; 4,152,338; 4,152,339; and British Application No. 2,019,839A. While the aforementioned prior art tends to bring about some improvement in the performance of the phosphorus-vanadium catalyst, there remains much room for improvement, particularly from the standpoint of high conversion, yield and catalyst life.
The object of the present invention is to provide a nonprecipitated method for the manufacture of phosphorus, vanadium and phosphorus-vanadium-co-metal oxide catalysts, by carrying out the reaction in both aqueous and non-aqueous solvents. A further object is to provide a process for the manufacture of maleic anhydride in the presence of the catalyst manufactured by the novel process.
Our catalyst is suitably prepared in aqueous solvents using organic acids. When using co-metals our catalyst is suitably prepared in aqueous solvents using inorganic acids. Otherwise the procedure is the same as described herein below. Suitable co-metals include molybdenum, zinc, uranium, tungsten, tin, bismuth, titanium, zirconium, niobium, chromium, and antimony which are introduced as their respective oxides. When the aqueous solution is clear, and substantial reduction of vanadium (V) to vanadium (IV) has taken place, phosphoric acid, such as 85 percent ortho-phosphoric acid, is added to form a soluble aqueous vanadium-phosphorus-metal oxide catalyst. A large quantity of water-hydrogen chloride is removed from the catalyst solution giving a thick syrup which is then diluted in a suitable alcohol having from about 1 to about 8 carbon atoms or other suitable organic solvent. An aromatic acid or an aromatic anhydride or a mixture of these is added to this alcoholic catalyst solution. Suitable alcohols are methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, and other alcohols containing from about 1 to about 8 carbon atoms. Suitable anhydrides are phthalic anhydride, trimellitic anhydride, pyromellitic dianhydride, benzoic anhydride, toluic anhydride, etc. Suitable aromatic acids are phthalic acid, trimellitic acid, benzoic acid, toluic acid, terephthalic acid, isophthalic acid, hemimellitic acid, trimesic acid, pyromellitic acid, 2,4-xylylic acid, isoxylylic acid, 2,6-xylylic acid, paraxylylic acid, mesitylenic acid, 2-ethylbenzoic acid, 3-ethylbenzoic acid, 4-ethylbenzoic acid, prehnitylic acid, durylic acid, mesitoic acid, pentamethyl benzoic acid, mellitic acid, etc.
In place of HCl, other acids capable of reducing vanadium can be utilized, such as HBr. Suitable metals include molybdenum, tungsten, zinc, uranium, chromium, tin, bismuth, zirconium, niobium, titanium, and antimony. Our catalyst has a much higher activity than catalysts of the prior art, such as those disclosed in U.S. Pat. No. 3,862,146, and U.S. Pat. No. 4,328,126. Our process recovers 100 percent of the vanadium feedstock, compared to the usual precipitative process which recovers only about 60 percent of the vanadium. Among the many advantages of our novel process for the manufacture of the catalyst can be cited the quantitative use of the expensive vanadium and the use of very cheap solvents, such as water and methanol or ethanol or aromatic acids or anhydrides and phosphoric acid.
The novel catalyst comprises a phosphorus-vanadium mixed oxide and a phosphorus-vanadium mixed oxide promoted by metals. The atomic ratio of the vanadium to phosphorus can suitably be in the range of 0.5:1 to 1.25:1.0. The total atomic ratio of co-metal to vanadium advantageously is in the range of 0.001.1 to 1:1. It is preferred that the total atomic ratio of molybdenum to vanadium should be in the range of 0.001:1 to 0.2:1. The atomic ratio of phosphorus to vanadium is suitably in the range of 0.8:1 to 2:1, preferably 1:1 to 1.5:1.
The co-metal, such as molybdenum, may be added as a compound together with vanadium, or separately introduced into the solution. Suitable molybdenum compounds comprise molybdenum oxide and most soluble molybdenum salts. If it is desired to improve physical properties of the catalysts, they may be treated with the suspension of an inert support; for example, alumina, titania, silicon carbide, kieselguhr, pumice, or silica. The catalyst may be reinforced with such materials at any stage in its preparation.
According to our process, the average valence of vanadium is in the range of about 3.8 to about 4.2. In our catalyst preparation, hydrous phosphoric acids or various anhydrous phosphoric acids may be used, including ortho-phosphoric, pyrophosphoric, triphosphoric acid or meta-phosphoric acid. Suitable vanadium compounds include: vanadium oxides, such as vanadium pentaoxide, vanadium tetroxide and the like; vanadium oxyhalides, such as vanadyl chloride, vanadyl dichloride, vanadyl trichloride, vanadyl bromide, vanadyl dibromide, vanadyl tribromide, and the like; vanadium-containing acids, such as meta-vanadic acid, pyrovanadic acid, and the like; vanadium salts, such as ammonium meta-vanadate, vanadium sulfate, vanadium phosphate, vanadyl formate, vanadyl oxalate, and the like. However, vanadium pentoxide is preferred.
This invention also comprises a process for oxidizing butane to maleic anhydride by contacting it in the presence of oxygen with the novel catalyst. The oxidation of butane to maleic anhydride may be accomplished by contacting n-butane in low concentration in oxygen with the described catalyst. Air is entirely satisfactory as a source of oxygen, but synthetic mixtures of oxygen and diluent gases, such as nitrogen, may also be employed. Air enriched with oxygen may be used.
The gaseous feedstream to the oxidation reactors will normally contain air and about 0.2 to about 1.7 mole percent of n-butane. About 0.8 to about 1.5 mole percent of n-butane is satisfactory for optimum yield of maleic anhydride for the process of this invention. Although higher concentrations may be employed, explosive hazards may be encountered. Lower concentrations of butane, less than about one percent, of course, will reduce the total yield obtained at equivalent flow rates and, thus, are not normally economically employed. The flow rate of the gaseous stream through the reactor may be varied within rather wide limits, but the preferred range of operations is at the rate of about 100 to about 4000 cc of feed per cc of catalyst per hour, and more preferably about 1000 to about 2400 cc of feed per cc of catalyst per hour. Residence times of the gas stream will normally be less than about four seconds, more preferably less than about one second, and down to a rate where less efficient operations are obtained. The flow rates and residence times are calculated at standard conditions of 760 mm of mercury at 25.degree. C. A variety of reactors will be found to be useful, and multiple tube heat exchanger-type reactors are quite satisfactory. The tops of such reactors may vary in diameter from about one-quarter inch to about three inches, and the length may be varied from about three to about ten or more feet. The oxidation reaction is an exothermic reaction and, therefore, relatively close control of the reaction temperatures should be maintained. It is desirable to have the surface of the reactors at a relatively constant temperature and to have some medium to conduct heat from the reactors, such as lead and the like, but it has been found that eutectic salt baths are completely satisfactory. One such salt bath is a sodium nitrate, sodium nitrite, potassium nitrate, eutectic constant temperature mixture. An additional method of temperature control is to use a metal block reactor, whereby the metal surrounding the tube acts as a temperature regulating body. As will be recognized by those skilled in the art, the heat exchanger medium may be kept at the proper temperature by heat exchangers and the like. The reactor or reaction tubes may be iron, stainless steel, carbon steel, nickel, glass tubes, such as vycor, and the like. Both carbon steel and nickel tubes have excellent long life under the conditions of the reaction described herein. Normally, the reactors contain a preheat zone under an inert material, such as one-quarter inch Alundum pellets, inert ceramic balls, nickel balls, or chips, and the like, present at about one-half ot about one-tenth the volume of the active catalyst present.
The temperature of reaction may be varied within some limits, but normally the reaction should be conducted at a temperature within a rather critical range. The oxidation reaction is exothermic, and once reaction is underway, the main purpose of the salt bath, or other media, is to conduct heat away from the walls of the reactor and control the reaction. Better operations are normally obtained when the reaction temperature employed is no greater than 20.degree.-50.degree. F. above the salt bath temperature. The temperature of the reactor, of course, will also depend to some extent upon the size of the reactor and the butane concentration.
The reaction may be conducted at atmospheric, superatmospheric, or below atmospheric pressure. The exit pressure will be at least slightly higher than the ambient pressure to ensure a positive flow from the reaction. The pressure of the inert gases must be sufficiently higher than the reaction temperature to overcome the pressure drop through the reactor.
Maleic anhydride may be recovered by a number of ways well-known to those skilled in the art. For example, the recovery may be by direct condensation or by absorption in suitable media, with specific operation and purification of the maleic anhydride. The following examples will serve to provide a fuller understanding of the invention, but it is to be understood that these examples are given for illustrative purposes only and will not be interpreted as limiting the invention in any way. In the examples the terms "conversion," "selectivity" and "yield" are defined as follows: